Wednesday, 8 February 2012

Newhaven Caspian Gull trait scores

Caspian Gulls in Sussex are often reported but with very few exceptions rarely are they fully documented.  On a high at having finally found one in the county that I am completely happy with (and have got reasonable images of) and the recent publication of a detailed British Birds article the temptation to try and rectify this was too great to resist.  Without digiscoping this bird may well have got away.  It would certainly have been hard to prove to the counties cynics (of which I'm one)!  Digiscoping doesn't have to be expensive if one has a telescope already and is only interested in a blog or getting images for identification purposes.  My camera cost £85 (although it is an unfashionable yellow!) and I made an 'adaptor' out of a pill box lid and a couple of washers costing 50p that works reasonably well most of the time.

Circumstances of observation. As reported on my blog, Frank Lambert and I found a first-winter (2CY) Caspian Gull about half-way along the West Arm of Newhaven Harbour on Sunday 5 February. It was present from at least 10.15-11.10am. Although it was rather distant, and the light poor for photography, the digiscoped images copied below show most of the necessary features. It was also seen briefly in flight, by both of us, and showed a very pale whitish underwing, white rump and solid black tail bar. John Cooper saw the same bird the following day when it, or a bird appearing similar, was also seen by Tony Cook, Matt Eade and Paul James.

Newhaven Caspian Gull when first picked out
Identification. This appeared a classic individual in every respect with a long, parallel sided predominantly dark bill, small white head with small forward placed dark eye, hind neck streaking, 4 coloured body (white head/underparts, grey mantle, brown coverts and black primaries), all dark tertails with white thumb-nails, long this legs and attenuated rear. It also had the ventral bulge, long primary projection and in flight a very pale/whitish underwing, white rump and black tail bar. However an apparent gull ‘expert’ is alleged to have expressed concerns about it not having sufficient grey on the mantle for this stage of the winter for it to be a Caspian Gull.   This, if true, is nonsense in relation to the bird photographed here if a quick internet check revealing similarly plumaged birds photographed at this time of year is to be believed. Further, in Caspian Gulls “moult is suspended for the winter – most birds complete post-juvenile moult by October with no (or only exceptional) further replacement of feathers beyond this point. Moult commences again in spring from April onwards.” (British Birds 104:709-10). Photographs of a bird taken in March showing less grey on the mantle than the Newhaven Caspian Gull (British Birds 104:plate 420) and another taken in January showing more (British Birds 104:plate 421) add weight to this as once a bird suspends its moult (by October) it will remain more or less unchanged until April (bar any normal wear).

The Newhaven Caspian Gull – a classic in every respect. Long, parallel sided predominantly dark bill, small white head with small forward placed eye, hind neck streaking, 4 coloured body (white head/underparts, grey mantle, brown coverts and black primaries), all dark tertails with white thumb-nails, long this legs and attenuated rear. It also had the ventral bulge, long primary projection and in flight a very pale/whitish underwing, white rump and black tail bar.
 
Trait scoring. Several highly competent observers have expressed no concerns at all with this birds identification, considering it a classic, but would it stand up to the latest criteria – trait scoring? Identification of Caspian Gull – part 2 by Gibbins, Neubauer & Small (British Birds 104:702-742) is the essential reference for this analysis.

For first-winters seen between October and March the above paper provides a system of scoring based on certain ‘traits’ to differentiate most pure Caspian Gulls from Hybrids. When added up these trait scores will indicate whether a bird is a Caspian Gull (combined score 12-25), Hybrid (22-32) or Herring Gull (29-37). Birds in the overlap zone are best left indeterminate. For a safe Caspian Gull identification a score below 22 is required although the lower the score the better.

Trait 1. Extent of scapular moult. On this bird no first-generation feathers are present. This feature is exhibited in 92% of Caspian Gulls but only 13% of Herring Gulls and 78% of hybrids (based on samples of 63, 85 and 12 respectively). Score = 0.
Newhaven Caspian Gull showing all second generation scapulars.

Trait 2. Greater-covert pattern. This bird had white edges with delicate notches or vermiculations or dark brown centre with white tip to 1/3rd of length. This is shown by 63% of Caspian Gulls, 0% of Herring Gulls and 12% of Hybrids. Score = 1.
Newhaven Caspian Gull showing delicate greater-covert pattern. This also shows the very long, thin legs. Not a trait feature for first-winters but it is for adults where it would score 0.
 
Trait 3. Ventral bulge. This bird showed a ventral bulge, as did 62% of Caspian Gulls, 14% of Herring Gulls and 34% of Hybrids. Score = 0.
Newhaven Caspian Gull showing ventral bulge

Trait 4. Primary projection. This bird showed a very long primary projection (>0.6). Consistent with 72% of Caspian Gulls but only 6% of Herring Gulls and 56% of Hybrids. Score = 0.
Newhaven Caspian Gull showing primary projection of about 0.65 (roughly 6/9). Note also small head, forward placed eye and parallel sided bill, all features associated with Caspian Gull but not ‘traitable’. This image also shows the long thin legs, attenuated rear (with long primary projection) and 4-toned plumage. In every way the classic Caspian Gull!

Trait 5. Greater-covert moult. Scored from 0-5 depending on the extent of moult (0 almost all, 5 none). Images of this bird and my ability to recognise replaced feathers make this very hard to assess. I can perhaps excuse my incompetence in this area as by early February all second generation feathers would be 4-6 months old and so subject to some wear. It appears that none have been replaced which would be consistent for 35% of Caspian Gulls, all Herring and 78% of Hybrids and would give a score of 5. 
Newhaven Caspian Gull appearing to show all first generation greater coverts
 
Trait 6. Median-covert moult. Scored from 0-5 depending on the extent of moult (0 almost all, 5 none). As above, images of this bird and my ability to recognise replaced feathers make this very hard to assess, more so as second generation feathers would be 4-6 months old, but juvenile median coverts would be darker with more contrasting centres so many may have been replaced. Assuming the worst, that none have been replaced, would be consistent for 19% of Caspian Gulls, 99% of Herring and 67% of Hybrids. This would give a score of 5 although its true score is probably lower?
Newhaven Caspian Gull not really showing enough detail of median coverts so the worst trait score was assumed (that no feathers have been replaced) although some might be second generation as replaced feathers will be 4-6 months old and so perhaps not stand out that well. The small head and protruding breast show well in this image.

Trait 7. Tertial moult. Scored from 0-3 depending on the extent of moult (0 three or more new tertials, 3 none). Hard to judge but all tertials are assumed to be old. This is consistent with 66% of Caspian Gulls, 100% of Herring Gulls and 89% of Hybrids. Score = 3. 
Newhaven Caspian Gull showing what appear to be old tertials. Also evident the dark tail bar and impressions of the sides of a white rump.

Trait 8. Darkness of body and head. This bird has reduced greyish wash or streaking confined to flanks and single streaks around the neck. 41% of Caspian Gulls show this but none of the sampled Herring Gulls or Hybrids. Score = 1. 
Newhaven Caspian Gull showing reduced streaking confined to flanks and nape, although a score of 0 is almost deserved in this image. Also long legs, small head, predominantly all dark bill and protruding breast are noticeable in this image.
 
Trait 9. First generation tertial pattern. Shows a diffuse white tip like a Common Gull. Consistent with 65% of Caspian Gulls, 1% of Herring Gulls and 33% of Hybrids. Score = 0.
Newhaven Caspian Gull showing thumb-nail white tips to tertials

Trait 10. Scapular pattern (second generation). Shows strong, contrasting shaft streaks on less than half of feathers. Consistent with 30% of Caspian Gulls but none of sampled Herring or Hybrids. Score = 2. 
Newhaven Caspian Gull showing contrasting shaft streaks on less than half scapulars with the odd diamond-shaped dark centre just about visible in this image
 
Trait summary. The above trait scores, assuming the worst for covert moult (traits 5 & 6), total 17. An average median-covert score reduces it to 14. The average overall trait score for first-winter Caspian Gulls in the sample was 18.2, within the range 12-25. The range for Hybrids is 22-32. This bird, even with the worst median-covert score has a trait score below the average for the species putting it towards the more distinctive end of the range. This is no surprise given its classic appearance.
Newhaven Caspian Gull - the complete package.
 
Non-traitable features. Other features noted that are not 'traitable' were the whitish underwing, white rump and dark tail band (seen by both Frank and myself in a brief flight view but sadly not documented), an almost completely dark bill, bill shape (L:D ration of about 3, trait score = 0 were it an adult), small head with small forward placed eye and bulging upper breast. Most of these features are evident in the following images.
Caspian Gull at Newhaven showing a suite of classic features, in particular its very long thin stilt-like legs, protruding breast, small head and forward placed eye

Caspian and Herring Gulls at Newhaven Harbour.  Again the Caspian Gull shows a suite of classic features. Its relatively small size suggests that it is probably a female although this might be over-emphasised as the second gull from the right appears to be a good contender for an adult argentatus Herring Gull with its dark mantle, white primary tips and menacing size.

Caspian Gull at Newhaven showing a suite of classic features, in particular its small head, forward placed eye, lack of appreciable eye mask and in this view a bulging upper breast and ventral bulge

Newhaven Caspian Gull showing a suite of classic features, this angle showing its attenuated rear end

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